Expansion rock bolts are installed by drilling a bore into the rock strata, inserting the rock bolt into the bore and expanding the expansion mechanism within the tube of the bolt. Expansion rock bolts include an elongate tube, which is usually split longitudinally, with an expander mechanism positioned within the tube, normally towards the leading end of the tube that is inserted first into the drilled bore in the rock strata or wall. The expander mechanism is connected to a flexible cable or solid bar that extends to the trailing end of the rock bolt and attaches to an anchor. Expansion of the expansion mechanism is effected by pulling or rotating the cable or bar.
The bore that is drilled into the rock strata is intended to be of a smaller diameter than the outside diameter of the tube, so that the tube is already a friction fit within the bore prior to expansion of the expansion mechanism. This maximises frictional engagement of the rock bolt with the bore wall. This method of insertion is relatively simple and is in contrast with other forms of rock bolts that employ resin or grout to anchor the rock bolt within the bore.
The simplicity of installation of expansion rock bolts is in contrast with installation of resin anchored bolts, in which a resin cartridge is usually employed. The resin cartridge is required to be inserted into the bore prior to the bolt being inserted therein. Insertion of the resin cartridge is sometimes very difficult, because typically the tunnel walls extend to a significant height, so that access to bores into which the cartridge is to be inserted can be inconvenient. Additionally, the resin which is employed is relatively expensive and has a limited shelf life.
Cement grouted rock bolts are less expensive than resin anchored bolts, but application of the cement is more cumbersome than that of the resin. Cement grouting requires cement mixing equipment, as well as pumping and delivery equipment, to deliver the mixed cement into the bore.
Resin or cement anchored rock bolts generally anchor in a bore to provide greater levels of rock reinforcement or stabilisation as compared to friction rock bolts, because such bolts usually have a better bond between the bore wall and the resin or cement, compared to the frictional engagement of a friction rock bolt. Also, particularly for cement anchored rock bolts, there is a proper bond along the full length of the rock bolt and the bore wall. However, the advantages of speed of installation and cost make friction rock bolts attractive in suitable environments.
Any form of rock bolt is liable to fail if the bolt is exposed to excessive loading by the rock strata into which the bolt has been installed. Failure can be tensile or shear failure or it can be a combination of tensile and shear failure. In expansion rock bolts, the bolt can fail through fracture of the tube. Failure of that kind can often be tolerated provided the bar or cable of the bolt does not fail also. However, if the rock bolt is loaded to the extent that both the tube and the bar or cable both fail, then there is the potential that a section of the rock bolt that is towards the open end of the bore (the trailing end of the rock bolt) can eject from the bore with considerable momentum, posing a danger to workers and equipment within the immediate vicinity. The section of the rock bolt that can eject from the bore can include a portion of the tube and the bar or cable, the anchor mechanism that attaches to the trailing end of the cable or bar, and the rock plate. Sometimes other accessories can also be included in the ejected section.
It will thus be readily apparent that prevention of an ejection of the above kind would be desirable.